The D line is probably more easily seen in a colored flame made by adding salt water to an alcohol-burner flame: https://en.wikipedia.org/wiki/Emission_spectru

The D line is probably more easily seen in a colored flame made by adding salt water to an alcohol-burner flame: https://en.wikipedia.org/wiki/Emission_spectrum#/media/File:Cmglee_Cambridge_Science_Festival_2016_sodium_lines.jpg https://en.wikipedia.org/wiki/Emission_spectrum#/media/File:...